ELECTRICAL METHODS 457 



EQUIPOTENTIAL POINT AND LINE METHODS 



In equipotential point and line methods, artificially created potential 

 fields are utilized. In favorable cases, the use of these methods permits 

 mapping subsurface deposits of anomalous conductivity, such as a highly 

 conductive ore body in a less conductive formation. 



OPERATING PRINCIPLE 



When an electric potential is applied between two points, or between 

 two parallel line conductors, on the surface of the ground, an electric 

 current will flow. The potential distribution produced by flow of electric 

 current in a homogeneous medium can be readily calculated. (See Figure 

 285.) Where the ground is not homogeneous, the potential distribution 

 will not follow the pattern calculated for the homogeneous medium. 

 Hence, it is always possible, at least theoretically, to detect the presence 

 of an inhomogeneity by comparing the measured potential distribution 

 with the theoretical distribution for a homogeneous medium. 



In practice the usual procedure is to note any deviation of the potential 

 distribution from a regular pattern and to attribute this to subsurface 

 inhomogeneities, without recourse to comparison of theoretical and 

 observed distributions — a form of visual interpretation qualitative in 

 nature. 



The most practical type of energizing current is direct current or low 

 frequency alternating current. From a theoretical viev/point, the concept 

 of equipotential lines in the case where alternating fields are used has sig- 

 nificance only with reference to mean efifective values of the potential. 

 However, for low frequencies the field distribution is represented with 

 sufficient accuracy by the D.C. field distribution. 



When the frequency is not sufficiently low, appreciable phase shifts 

 will occur. The phase shifts are produced by the inductive effects of the al- 

 ternating field in the conductive materials, e.g., the ground, mineral bodies, 

 et al, lying within the area investigated. When alternating current potential 

 profiles are to be determined in regions of high conductivity where appre- 

 ciable phase shifts occur, use is made of some form of phase compensator, 

 as described later. 



D.C. galvanometers and potentiometers or A.C. galvanometers and 

 vacuum tube voltmeters may be used with direct current or low frequency 

 alternating current, respectively, for determining the lines of equipoten- 

 tial. If audio-detecting means (usually headphones with vacuum tube 

 amplification) are used, the frequency should only be high enough to give 

 a clear cut signal, because the effective depth of penetration decreases as 

 the frequency increases. Frequencies from 100 to 500 cycles are often 

 employed for work of this type. 



The potential methods are chiefly useful for general reconnaissance 

 purposes. In simple cases, such as sulphide lodes extending to within 50 



